Black chromium plating process and composition

ABSTRACT

BLACK CHROMIUM PLATE ARTICLES ARE PRODUCED BY THE USE OF A NOVEL ELECTROPLATING BATH, WHEREIN A LIGHT AND HEAT ABSORBING DECORATIVE AND PROTECTIVE ELECTROPLATE IS APPLIED TO A BASIS METAL OR OTHER CONDUCTIVE SUBSTRATE. THE BATH COMPRISES CHROMIC ACID, SODIUM OR POTASSIUM IONS, FLUOSILICATE IONS AND AN ORGANIC REDUCING AGENT.

United States Patent 3,723,261 BLACK CHROMIUM PLATING PROCESS ANDCOMPOSITION Robert D. Byers, Syracuse, N.Y., and Winslow H. Hartford,Charlotte, N.C., assignors to Allied Chemical Corporation, New York,N.Y.

No Drawing. Filed Oct. 30, 1970, Ser. No. 85,701 Int. Cl. C231 /06, 5/48U.S. Cl. 204-51 14 Claims ABSTRACT OF THE DISCLOSURE Black chromiumplated articles are produced by the use of a. novel electroplating bath,wherein a light and heat absorbing decorative and protectiveelectroplate is applied to a basis metal or other conductive substrate.The bath comprises chromic acid, sodium or potassium ions, fluosilicateions and an organic reducing agent.

(1) organic coatings;

(2 chemical treatments to product (a) oxide coatings and (b)precipitated films; and

(3) electrolytic treatments both anodic and cathodic.

The deposits obtained by the cathodic processes offer considerableopportunity for varying the characteristics and properties of thecoating inasmuch as these properties are related to readily controllablefactors such as time, temperature, current density and composition ofthe electrolyte. Furthermore, the most durable of all theelectrodeposited black finishes are probably the chromium base platessince they have fairly good corrosion resistance and their mechanicalproperties are superior to those of many other black finishes.

Chromium coatings such as those produced by this invention may be usedas coatings for food containers and the tin-free sheet stock (TFS) fromwhich they are fabricated because the deposits are corrosion resistantand provide a surface receptive to the application of coatings such aspaint, lacquer and the like. Black chromium electrodeposits find use asdecorative black finishes, for example, on metal furniture, instrumentcases, automobile and appliance parts, and plumbling supplies Wheretheir corrosion resistance coupled with their appearance makes themsuperior to other black finishes, such as paint. More specifically,these electrodeposited coatings find use in areas where their heat andlight absorbing properties are important, as for example in themanufacture of non-reflective safety automotive trim, cameras andoptical equipment, and in the production of non-reflective militaryhardware such as firearms, ordnance parts, communication equipment andmilitary uniform fastenings. In the past it has been the practice toproduce black chromium electrodeposits by employing such plating bathsas those containing chromic acid (Cro acetic acid and fluoride-bearingions. These are exemplified in U.S. Pat. 1,975,239 and British Pat.408,097. However, it has been found that in using these baths it wasnecessary to operate them at temperatures aslow as 50 F., and to employhigh current densities, in the order of 900 to 2,000 ampere's per squarefoot (a.s.f.). To successfully control and distribute such high currentdensities,

3,723,261 Patented Mar. 27, 1973 and at the same time maintain therelatively low bath temperatures required, Was both costly anddifficult.

Many of the methods disclosed in the prior art require the restrictionor exclusion of the sulfate ion, which is replaced as a catalyst byeither one or more of acetate, vanadate, nitrate, urea, fluosilicate,fluoride, fiuoborate or the sulfamate ion. U.S. Pat. 2,824,829, forexample, discloses a bath containing in addition to chromic acid,relatively small amounts of a soluble vanadium salt such as ammoniumvanadate, and a carboxylic acid such as acetic acid. Here again a highcurrent density is employed (400-2000 a.s.f.) and control is diflicultand costly.

The plating bath disclosed in U.S. Pat. 2,623,847 requires the totalabsence of the sulfate ion in the plating bath. Since sulfate is presentas a contaminant in essentially all commercial grades of chromicanhydride, it first must be precipitated and removed, generally asbarium sulfate.

The application of the common ion effect to render a chromium platingbath self-regulating with respect to the contained catalyst is not newand has generally been employed to regulate the concentration of thesulfate ion or the fluosilicate ion, since these ions are frequentlyused as the catalyst. Usually a mixed catalyst has been used employingthe potassium ion (K+) as the regulator for the fluosilicate ion (SiFand the strontium ion (Sr++) as the regulator for the sulfate ion (S05).In connection with the development of the self-regulation concept withrespect to the catalysts employed in chromium plating baths, we refer toU.S. Pats. 2,042,611; 2,640,021; 2,640,022; 2,950,234; 2,952,590 and3,303,114. Although these self-regulating chromium plating electrolytesare designed to maintain the catalyst at a level such as to provide abath capable of producing a uniform plate (usually a bright plate), noneof these are designed to hold a chromium plating bath within the limitsnecessary to provide an adherent black chromium plate.

One of the most satisfactory black chromium plating baths of the priorart, and one which may be employed to produce black chromium depositshaving excellent characteristics, is the so-called Graham-Pinkertonbath. (Graham, A. K., Proc. Am. Electroplaters Soc., 46, 61 (1959); U.S.Pat. 2,985,567, H. L. Pinkerton, 5-23-61.) This bath may becharacterized as follows:

Although this bath is more easily controlled than many otherformulations, it is still extremely sensitive to sulfate contamination,catalyst ratio and temperature. No more than 0.01 g./l. of sulfate ioncan be tolerated in this bath.

It is an object of the present invention to provide an improvedelectroplating bath composition for the deposition of blackchromium-containing electrodeposits.

It is a further object of the present invention to provide anelectroplating bath and process for electroplating an adherent blackchromium-containing coating, in which process the concentration of thefluosilicate catalyst is automatically controlled.

It is another object of the present invention to provide a process forelectrodepositing an adherent, black chromium coating wherein minoramounts of impurities, such as sulfate ions in amounts up to about .02%based on the Cr content, can be tolerated in the electrolyte withoutadversely affecting the coating.

It is still a further object of this invention to provide corrosionresistant plated articles having a black, adherent chromium and chromiumoxide electroplate to which other coatings may subsequently be applied.

These and other objects are accomplished according to our inventionwherein an adherent black chromium coating is electrodeposited on abasis metal from a bath having the following composition:

BLACK OI-IROMIUM PLATING BATH [Composition and operating conditions]Range Preferred ClOs (g./l.) 300-500 350-450 N a+ (g./l.) 6-52 10-35Ratio CrO; to Nat 10:1-50z1 13:1-35:1

or K+ (g./l.) 10-00 17-60 Ratio CrO to K 5. :1-30zl 7. 5:l20:1 S01(g./l.) 00.l0 0-0.08 Fa (g 0. 2-3. 0 0. 4-1. 0 An organic r ucing agentsuch (g./l.) 1.0-3.0 1.5-2.5 Temperature F.) (SO-100 70-90 Current,density (a.s.f.) -500 100-400 In general, high Na or K ion concentrationshould correlate with high CrO concentration, as indicated by the aboveratios. The Na or K cations can be added as their hydroxides or combinedwith other compatible anions. For example, they can be introduced inpart by employing sodium or potassium chromates or bichromates ratherthan chromic anhydride, and/ or sodium or potassium fluosilicate ratherthan fluosilicic acid.

As indicated above, the weight ratio of the CrO to the Na ions fallsbetween 10:1 to 50:1 for the general range, and between 13:1 and 35:1for the preferred range. Correspondingly, the ratio of CrO to the K ionsfalls between 5.5:1 and 30:1 for the general range while the preferredrange lies between 7.5 :1 and 20:1.

It was unexpectedly found that the above bath will produce a smooth,black adherent chromium deposit and has a considerably greater tolerancefor sulfate than do typical electrolytes for deposition of the blackchromium plate of the prior art. Also, it was unexpectedly found that noappreciable brightening of the electroplate followed the addition offluosilicic acid in excess of 1.0 g./l. as would normally be expectedwith an excess of catalyst ion. The fact that a satisfactory blackchromium coat can surprisingly be obtained, even in the presence of anexcess of fiuosilicic acid, is believed attributable to the fact thatthe bath of the present invention is self-regulating with respect to thecatalyst ion. This self-regulation is brought about by the presence inthe bath of a high concentration of either sodium ions or potassiumions, which by common ion effect limits the solubility of the sodium orpotassium fiuosilicate in the bath. The effect is more striking with thepotassium ion than with the sodium ion, for K SiF is even less solublethan Na SiF The electroplating baths provided by the present inventionmay be characterized as containing chromic acid (CrO which need not betreated for the removal of traces of sulfate ion unless they exceed0.02% and a sodium or potassium ion concentration as follows: The sodiumion concentration should be at least 6 g./l., with a range to about 52g./l., (preferably between 10 and 35 g./l.), or the potassium ionconcentration should be at least 10 g./l. with a range to about 90g./l., (preferably between 17 and 60 g./l.). A Na+/CrO ratio of at leastabout 1:50 or a K+/CrO ratio of at least about 1:30 is necessary tomaintain the fiuosilicate ion within the range required for a blackchromium deposit.

Although the sodium and potassium ions are given as probably the twomost practical, any alkali metal ion not contraindicated by thecomposition of the bath or detrimental to the electroplate, thefluosilicate of which is sparingly soluble in the bath, may be employedas a catalyst regulator.

Although the sodium and potassium ions are preferably added in the formof their hydroxides, many other soluble sodium or potassium salts, notdetrimental to the electrolyte or the chromium plate, may be used tosupply at least part of the required sodium or potassium ion; forexample, the chromate, dichromate, carbonate, bicarbonate orfiuosilicate of sodium or potassium.

Furthermore, although the fluosilicate ion is preferred as thefluorine-bearing catalyst, there are several others that may besimilarly employed, such as the fluoaluminate, fiuotitanate,fluozirconate, fluoride and fluoborate anions. To be suitable inchromium plating baths as a regulated catalyst, the first requirement isthat the chosen anion and its regulating cation be in no way detrimentalto the plating bath or the resulting plate; the second, that the salt ofthe catalytic anion and the regulating cation be sparingly soluble inthe plating bath so that control by common ion effect is possible.

The upper limit of the catalyst regulator of the present invention isnot critical, although 52 g./l. of Na+ or g./l. of K+ may be consideredan upper practical limit for depositing acceptable black chromiumdeposits. If a reduction in the SiF concentration is indicated by toogray or too bright a plate, the concentration of the Na or K ions shouldbe increased.

The sucrose is preferably included in the composition as an example ofan organic reducing agent which, through its action, insures thepresence of a small amount of trivalent chromium in the plating bath.Trivalent chromium ions are present during normal plating operations,and the presence of sucrose makes it possible for the plating process tobegin somewhat faster than would be possible without it. It serves toinitiate the plating process, for without it, the trivalent chromiumwould have to be formed from the chromic acid by electrolysis, beforeelectrodeposition of the chromium could begin. Other organic reducingagents could be used such as maltose, methanol, glycollic acid, glucose,and levulose, but sucrose is readily available and preferred.

In carrying out the process of our invention, preferably 350-450 g./l.of chromic acid (CrO containing no more than about 0.02% 80.; isdissolved to produce each liter of the plating bath. In the event thatmore than .02% S0 is present in the CrO or more than 0.10% g./l. of $0.;in the bath, it should be removed by precipitation with Ba++, as asatisfactory black plate cannot be obtained in the presence ofappreciably more than .02% S0 The bath also contains preferably 10 to 35g./l. of Na ion, 0.4l.0 g./l. of fluosilicate ion as catalyst, and 1.5to 2.5 g./l. of sucrose.

In practice, a basis metal which has been thoroughly cleaned so as to befree of traces of grease, is used as the cathode. The current densityemployed may vary between 40 and 500 a.s.f., preferably between and 400a.s.f., and the plating temperature may vary between 60 and 100 F.,preferably between 70 and 90 F. The above plating bath of the presentinvention will provide a smooth, non-reflective, adherent, blackchromium electroplate, the surface of which may provide a base forsubsequent coatings, such as electroplates, paints, lacquers or thelike. As noted above, an adherent, non-reflective, black chromium plateis obtained with this plating bath, even when the quantity offluosilicate in the bath is in excess of 2.5 g./l. and/or the sulfateconcentration present in the bath is up to 0.10 g./l. of 0.02% byweight, based on the weight of the chromic anhydride (CrO employed.Under either of theese circumstances, a grey or bright plate wouldnormally be obtained.

It is believed that the high sodium or potassium ion concentration inthe plating solution holds the concentration of the fluosilicate ions inthe plating solution to within permissible limits (about 0.2-1.2 g./l.,preferably 0.4- 1.0 g./l.). The excess SiF ions are precipitated as NaSiF or K SiF and the SiF catalyst concentration is held below about 1.0g./l., above which a bright plate may form. This self-regulating actionexemplified by good results regardless of whether or not an excess ofthe .fluosilicic acid catalyst has been added, is brought about by thehigh concentration of the sodium or potassium ions TABLE I.RESULTS OF 6by deep black color, excellent adhesion and good corrosion resistance.

EXAMPLES 1-7 Table I below tabulates seven runs made on Hull cell testpanels. The I-Iull cell provides a test panel in which the currentdensity varies smoothly over a wide range, by virtue of the placement ofthe cathode at a fixed angle with respect to the anode. The standardHull cell con- 10 tains 267 ml. of plating bath solution so that a 2gram addition corresponds to 1 oz./gal. (7.5 g./l.). The Hull cell andits operation are described in Modern Electroplating edited by RA.Loweheim, 2d edition, 1963, pp. 525-528, which disclosure isspecifically incorporated by reference.

HULL CELL TESTS ON SELF-REGULATING BLACK CHROME BATH [All on Brass HullCell Panels] Test number- 1 2 3 4 5 6 l 7 1 01'03, g./1 400 400 4 400400 400 400 Percent 80-; in CrO; 0.02 0. 02 0. 02 0. 02 0. 02 0. 02 0.02 Ba (OH), added to remove S04 Nag./l. (added as NaOH) 84. 5 34. 5 34.5 84. 5 34. 5 34. 5 34. 5 SiFa, g./l. (added as menu)- 1. 1. 2. 0 2. 52. 5 2. 5 2. 5 Sucrose, g.ll 2.0 2. 0 2. 0 2.0 2.0 2.0 2.0 Tern erature,F 68 68 68 68 77 86 95 Tota amperage, amps. 10 10 10 10 10 10 Time, mlns5 5 5 5 5 5 5 Blaek-plate range, amps/it. -420 45-420 45-420 45-42045-420 45-420 45-420 1 Some lightening of black. I Lightening of black.

in amounts up to about 0.05% by weight, although much is produced withno more than 0.02%. CrO with 0.02% S0 or less may be used in the bath ofthe present invention without any prior purification step, whereas inbaths of the prior art, eg US. Pat. 2,985,567, not more than 0.01% S0contamination based on the Q0 can be tolerated. It is thought that thetolerance of our bath to traces of S0 and small amounts of otherimpurities that may be present, can be attributed to the presence of anexcess of sodium ions in the bath of our invention.

The black chromium electrodeposit may be applied to any electricallyconductive substrate, particularly copper, iron, zinc, nickel, lead, orto alloys of any ,of these. Unlike the process of US. Pat. 2,985,567 andothers, a satisfactory black adherent chromium coatinng can be depositedon any of these, whereas in the method disclosed by the aforementionedpatent, a strike coat of nickel must be deposited on the article to bechrome plated before the chrome plate is applied, except where the basismetal is nickel. If, however, such a strike or flash coat is used inconnection with the process of the present invention,

an improved black-plating range will be obtained. Such strike coats maybe deposited from any Watts-type or other commercial nickel platingbath.

Black plates are commonly applied within 1-5 minutes at 100400 a.s.f.The plate so formed is probably about 002-006 mil in thickness. Thickerplates are undesirable.

Insoluble anodes, preferably of lead or lead alloys of the type used forconventional chromium plating, may be employed. Plain iron or steeltanks offer reasonable service life, for there is essentially nocorrosion problem at low temperatures and low SiF concentrations.

The process of the present invention has all the advantages of theaforementioned Graham-Pinkerton bath and improves upon it, in that thesulfate present in much of the available commercial chromic anhydridecan be tolerated. Also, unlike the Graham-Pinkerton bath, the presentbath is self-regulating with respect to the catalytic SiF and produces ablack chromium plate characterized In the tests here reported, the anodeis lead, and the plating bath contains less than 0.08 g./l. of sulfateion, and up to 2.5 g./l. of fiuosilicate ion. The Hull cell cathodes areof brass.

In each case, the baths produce a uniform, hard, smooth, adherentwear-resistant, black chromium electroplate. In Examples l-4, thetemperature of the bath is held at 68 F., whereas in Examples 5, 6 and7, the tem peratures are 77, 86 and F., respectively. In the last twoexamples there is a noticeable lightening of. the black coat produced,the effect being more pronounced at the higher temperature.

EXAMPLES 8-19 As with Examples 1 through 7, Examples 8 through 19 arerun in a Hull cell. Reference is made to Table II, below. In Examples 8,9, 10 and 11 the sodium ion content is 5.75, 11.5, 17.25 and 34.5 g./l.,respectively. It will be noted that the minimum cathode current densityrequired for black plate decreases as the amount of Na+ increases. Asatisfactory black plate is not obtained in any of these four runs at alow current density, however, as the 80.; content of the CrO used is inexcess of 0.03%, based on the CrO An identical set is run, consisting ofExamples 12, 13, 14 and 15, except in each case the S0 ion is removed bytreatment with 3 g./l. of BaCO The bath is stirred overnight before useand separated from the insoluble barium salts. A satisfactory, adherent,wear-resistant, black chromium electroplate is obtained in each instanceover a wide range of current densities.

Examples 16, 17 and 18 correspond with Examples 9, 10 and 11, exceptthat K+ in the amount of 9.75, 19.15 and 29.2 g./l., respectively, isused. These quantities are the equivalent of 14, 28 and 42 g./l. of KOH.As in the case of Examples 9, 10 and 11, the S0 content based on CrO isgreater than 0.03% and a black plate is not obtained. A black platecould only be obtained under these conditions by employing a highcurrent density.

TABLE IL-RESULTS F HULL CELL TESTS ON SELF-REGULATING BLACK CHROME BATH[All on Brass Hull Cell Panels Except as Noted] Example 19 correspondsto Example 14, except that 29.2 g./l. of K+ are used, and once again, asmooth adherent black chromium plate is obtained, although no betterblack plating range was observed than in Examp In the case of Examples8-14 and 16l9, the SiF present in the bath was determined in grams/literby analysis.

In each instance where a satisfactory, uniform, hard, smooth, adherent,wear-resistant, black chromium electroplate is obtained, a relativelylow cathode current density is used; hence, the process can be carriedout in conventional plating equipment. The chromic acid input can becontrolled by its density using a hydrometer. As previously noted, thecatalyst anion concentration is automatically controlled, but ifdesired, can also be determined and controlled through chemicalanalysis. The continued operation of the bath can be checked as to blackplating range by a Hull cell.

As many embodiments of this invention may be made without departing fromthe spirit and scope thereof, it is to be understood that the inventionincludes all such modifications and variations as come within the scopeof the appended claims.

We claim:

1. A composition of matter comprising an aqueous electrolyte consistingessentially of 300 to 500 grams per liter of CrO containing up to 0.02%by weight of S0 ion based on the CrO content, 0.2 to 3.0 grams per literof SiF ion, 1.0 to 3.0 grams per liter of a water soluble organicreducing agent, selected from the group consisting of maltose, methanol,glycollic acid, glucose, levulose and sucrose, and at least one memberselected from the group consisting of sodium and potassium ions, whereinsaid sodium ion is present in the bath in a concentra tion from 6 to 52grams per liter and in a weight ratio with respect to the CrO of from1:10 to 1:50, and said potassium ion is present in the bath in aconcentration from to 90 grams per liter and in a weight ratio withrespect to the CrO of from 1:5.5 to 1:30.

2. The composition of claim 1 wherein the SiF ion in the electrolyte isheld between 0.2 and 1.2 g./l.

3. The composition of claim 1 wherein the SiF ion in the electrolyte isheld between 0.4 and 1.0 g./l., and the S0 content does not exceed 0.08g./l.

4. The composition of claim 1 wherein the electrolyte contains 350 to450 grams per liter of CrO and said sodium ion is present in the bath ina concentration from 10 to 35 grams per liter and in a weight ratio withrespect to the CrO of between 1: 13 and 1:35.

5. The composition of claim 1 wherein the electrolyte contains 350 to450 grams per liter of CrO and said potassium ion is present in the*bath in a concentration from 17 to 60 grams per liter and in a weightratio with respect to the CrO of between 1:7.5 and 1:20.

6. The composition of claim 1 wherein the organic reducing agent issucrose.

Example number 8 9 10 11 2 13 14 15 1 16 17 18 10 CI'OJ g.[l 400 400 400400 400 400 400 400 400 400 400 400 Percent 504 in CrOs 0. 03 0. 03 0.03v 0. 03 0- 03 0. 03 0. 03 0. 03 0. 03 0. ()3 0. 03 0. 03 Ba (OH);added to remove S04... YOS YES :Ycs Yes Yes Na+, g./l. (added as NaOH)5. 11. 5 17. 2o 34. 5 5. 75 11. 5 11. 25 34. 5 SiFF, glll. (added asHgSiFg) 2. 0 2. 0 2. 2. 0 2- O 2- 0 2. 0 2. 0 2. 0 2. 0 2. O I 2 0 K+,g./l. (added as KOH) n 9. 75 19. 5 20. 25 2.). as Sucrose, g./l 2. O 2.0 2. 0 2. 0 2. 0 2. 0 2. 0 2.0 2.0 2. 0 2. 0 2. 0 Temperature, F 77 7777 77 77 77 77 77 77 77 77 77 Total amperage, amp 10 10 1(5) 1(5) 1g 1g12 1(5) 12 1(5) 12 12 hin e, amps/1H-.. 400 360 320 250 150-400 120-450100 100 180-420 380 300 270 300 SiFF, g.p.l. (analysis) 1. 00 0. 97 0.73 56 0- 81 0- 0. 66 0. 94 0. 47 O. 44

1 This bath on Ni.

15 7. A method of electrodepositing an adherent black chromium coatingon an electrically conductive substrate, which process comprises thesteps of making said substrate to be plated the cathode in an aqueouselectrolyte consisting essentially of 300 to 500 grams per liter of CrOcontaining up to 0.02% by weight of 50.; ion based on the CrO content,0.2 to 3.0 grams per liter of SiF ion, 1.0 to 3.0 grams per liter of awater soluble organic reducing agent, selected from the group consistingof maltose, methanol, glycollic acid, glucose, levulose and sucrose, andat least one member selected from the group consisting of sodium andpotassium ions, wherein said sodium ion is present in the bath in aconcentration from 6 to 52 grams per liter and in a weight ratio withrespect to the CrO of from 1:10 to 1:50, and said potassium ion ispresent in the bath in a concentration from 10 to grams per liter and ina weight ratio with respect to the CrO of from 1:5.5 to 1:30, passing adirect current between the anode and cathode to provide a cathodecurrent density of from 40 to 500 a.s.f. while maintaining thetemperature of said electrolyte between 60 and F.

8. The method of claim 7 wherein the sodium and potassium cations areadded in combination with any water soluble anion compatible with theelectrolyte and said method.

9. The method of claim 7 wherein the sodium and potassium cations areadded as their hydroxides.

10. The method of claim 7 wherein the sodium and potassium cations areadded wholly or in part as their fluosilicates.

11. The method of claim 7 wherein the sodium and potassium cations areadded wholly or in part as their chromates.

12. The method of claim 7 in which the direct current provides a cathodecurrent density of 100 to 400 a.s.f.

13. The method of claim 7 in which the plating temperature is maintainedbetween 70 and 90 F.

14. The method of claim 7 in which the aqueous electrolyte contains 350to 450 grams per liter of CrO 0.4 to 2.5 grams per liter of SiF ion, 1.5to 2.5 grams per liter of sucrose and at least one member selected fromthe group consisting of sodium and potassium ions wherein said sodiumion is present in the bath in a concentration from 10 to 35 grams perliter and in a weight ratio with respect to the CrO of from 1:13 to1:35, and said potassium ion is present in the bath in a concentrationfrom 17 to 60 grams per liter and in a weight ratio with respect to theCrO of from 1:7.5 to 1:20.

References Cited UNITED STATES PATENTS 3,157,585 11/1964 Durham 204-413,475,294 lO/1969 Seyb et a1. 204-51 3,511,759 5/1970 Nelson 204-51FREDERICK C. EDMUNDSON, Primary Examiner

